You will see "0 followers" and "0 following" on his page, click any of those. In the address bar, you will see a string of numbers and letters starting with npup - his public key. Copy it, add @ at the beginning and you can tag him in posts. Fetching his profile requires him interacting with the post on the other side though
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you can also get the pub key from the 3 horizontal dots in the top-right (not quite completely to the right)
